AND PHTSIOLOGT OF THE TUNICATA. 339 



look to immature specimens to be satisfied of this. When the 

 young Glavelina is about one twentieth of an inch long, and when 

 the thoracic portion would scarcely be half that length, the thorax 

 is remarkably like the embryo of Salpa. In profile both have a 

 subtriangular form, the anterior opening being placed at the angle 

 in front, and the posterior at an angle situated considerably back- 

 ward, the young of Glavelina having the two orifices nearly as far 

 apart as they are in the embryo of Salpa. At this early period 

 of growth the endostyle is distinctly developed in both forms, and 

 the ganglion and the oral lamina are clearly indicated, also the 

 ciliated band or anterior collar. So far, everything is alike ; the 

 general similarity of the respiratory cavity is obvious enough ; and 

 were the nucleus in Salpa produced a little more than it is back- 

 wards, the resemblance of the two would be almost complete. But 

 in the young of Glavelina there are, in addition to what has already 

 been described, the tentacular filaments of the incurrent tube, 

 which are now of considerable size ; and the branchial sac has 

 already commenced its development. 



The latter organ, however, is in an exceedingly rudimentary con- 

 dition : only a single transverse channel or primary vessel on each 

 side of the great ventral channel has made its appearance, and 

 does not yet extend much more than halfway across the thoracic 

 or respiratory cavity, on its way (so to speak) to the dorsal channel 

 in connexion with the endostyle. On each side of these growing 

 primary vessels five or six secondary vessels, at right angles to 

 them, have commenced to sprout ; and the distal extremities of all 

 of them, as well as of the primary vessels themselves, open through 

 the lining membrane of the thoracic cavity into the sinus-system 

 between it and the inner tunic. Thus is defined, on either side, 

 the nascent atrium, which is only an extension of the cloaca that 

 had been previously formed. Shortly another primary vessel makes 

 its appearance, extending from the ventral channel and connected 

 laterally with the extremities of the secondary vessels already 

 formed ; and then another series of secondary vessels is developed, 

 and afterwards another primary vessel, and so on, gradually in- 

 creasing the length of the two branchial leaflets (if they maybe so 

 called), which at the same time grow in breadth, passing further 

 and further across the thoracic cavity until at length they reach 

 the sides of the endostyle ; all the while the primary and secondary 

 vessels along the margins of the growing organ, open into the 

 pallial sinuses in the manner already indicated ; so that the boun- 

 dary of the water-space or atrium is well defined, and is always 



